It is well known that optical energy can be absorbed in a semiconductor material if the photon energy is greater than the band-gap energy of the semiconductor material. This phenomenon, known as the photovoltaic or photoconductive effect, occurs when the photons absorbed by the semiconductor material generate electron-hole pairs that produce a potential difference or increased conductance across the p-n junction of the semiconductor. The phenomenon has been used in the prior art to create a variety of hybrid optical/electrical devices. For a more detailed explanation of this phenomenon and its application, reference is made to J. Wilson and J. Hawkes, Optoelectronics: An Introduction, pgs. 286-327, Prentice Hall (1983).
Most well known among the uses of the photovolatic/photoconductive effect is the use of a photodiode for generating electrical power, e.g., solar cells converting sunlight to electricity. Other variations of the basic photodiode include the avalanche photodiode and the phototransistor, both of which internally amplify the current flow across the p-n junction of the photodiode. The photodiode is also used as a photodetector for detecting the presence or absence of optical energy, e.g. the light beam switch in an elevator door or a photochopper wheel. Optoisolators make use of the photodiode and a photoemmissive device (e.g., a light emitting diode or LED) to convert electrical energy to photon energy and back again for the purpose of decoupling a power source or an electrical signal. For example, U.S. Pat. No. 4,695,120 shows the combined use of optoisolators to electrically isolate all of the signals to an integrated circuit and a photodiode to provide the electrical power for the integrated circuit. A detailed description of the various types of optoelectronic devices that are available in the prior art is provided in Optoelectronics Fiber-Optic Applications Manual, Hewlett Packard (1981), and Optoelectronics: Theory and Practice, Texas Instruments (1978).
Another phenomenon that has been put to use in optical and hybrid optical/electrical circuit devices is the atomic level relationship between electrical fields and optical transmisivity, sometimes referred to as photorefractivity. Photorefractive substances exhibit a change in their index of refraction in response to the application of an electrical field. The most well known of photorefractive materials is the liquid crystal display or LCD. For a more detailed explanation of this phenomenon and its application, reference is made to Photorefractive Materials and Their Applications, Topics in Applied Physics, Vols. 61 and 62, Gunter, P. and Huignard, J. (eds.) (1989).
For purposes of understanding the wide variety of electrical/optical devices that are available in the prior art with respect to the present invention, it is helpful to categorize present hybrid electrical/optical circuit devices based upon the nature of their inputs and outputs. Primary electrical/optical devices convert photon energy (input) to electrical energy (output) or vice-versa. Examples of primary types of hybrid electrical/optical devices include the photodiode (optical input/electrical output), the light emitting diode (electrical input/optical output) and the semiconductor laser (electrical input/optical output). Intermediary or secondary electrical/optical devices have a common input and output, but use either photon energy or electrical energy as part of an intermediary step internal to the device. Examples of intermediary or secondary types of hybrid electrical/optical devices include solid state image intensifiers and electroluminiscient devices (optical input/output, electrical intermediary) and photoisolators and optocouplers (electrical input/output, optical intermediary).
Of interest for purposes of the present invention are those secondary or intermediary hybrid electrical/optical devices that utilize photorefractive materials as part of the intermediary step. Prior art application of photorefractive materials to hybrid electrical/optical devices has been limited to secondary devices having optical inputs and outputs with an electrical intermediary. The most prevalent uses of photorefractive materials include optical amplifiers, waveguides and light valves, such as liquid crystal light valves, which are used as part of an optical computing network. For example, U.S. Pat. No. 4,764,889 describes the use of optically nonlinear self electro-optic effect devices as part of an optical logical arrangement. U.S. Pat. No. 4,818,867 describes the use of an optical shutter on the output of an optical logic element. An overview of the various types of hybrid electrical/optical devices used in connection with prior art optical computing networks is provided in Feitelson, D., Optical Computing (1988).
Although the use of photorefractive materials is well known as part of the intermediary step for electrical/optical hybrid devices having optical inputs and outputs with an electrical intermediary, photorefractive materials have not been used in connection with other types of electrical/optical hybrid devices having electrical inputs and outputs with an optical intermediary. The optical intermediaries of photoisolators and optocouplers are designed for the optimum transfer of photon energy between the photoemissive device and the photovoltaic/photoconductive element and, hence, there is no need for intermediary optical control in such devices. Accordingly, it would be desirable to provide an optoelectronic device that makes use of photorefractive materials as part of an optical intermediary for electrical/optical hybrid devices having electrical inputs and outputs that could take advantage of a modulated transfer function of the photon energy in such a device.